An electronic apparatus is generally equipped with a power supply having a specific voltage, e.g., a battery in the electronic apparatus is able to supply voltages of 3.9-4.5V. However, various modules of the electronic apparatus require different supply voltages, e.g., an analog power amplifier may request a power voltage of 3.5V, and a digital processing module may require different power voltages such as 1.8 v and 5V. In order to ensure normal operations of the various modules in the electronic apparatus, a voltage converter is typically required to convert a direct current (DC) voltage level (e.g., a voltage from the battery) to another different DC voltage required by the various modules, namely, to convert a specific input voltage Vin to a different output voltage Vout.
In an existing voltage converter, for example, electric energy at the input end is temporarily stored in an inductor and/or a capacitor (i.e., performing a charging process), and then released at the output end at different voltages (i.e., performing a discharging process), thereby the input voltage Vin is converted to a required output voltage Vout. The charging process and discharging process are controlled by a control element (such as a switch), which is driven by a driving signal. As an example, the charging process corresponds to a conducting time when the driving switch is turned on for charging, and the discharging process corresponds to an open time when the driving switch is turned off for discharging. The conducting time corresponds to a pulse width of the driving signal.
Although an ideal voltage output of the voltage converter is a DC voltage, the charging and discharging processes therein result in a subtle fluctuation in the actual output voltage, which is shown in the frequency domain where the output voltage is not an ideal zero frequency component but possesses different frequency components. However, when a electronic module powered by the voltage converter includes a radio frequency circuit used for transmitting a radio frequency signal, the output voltage of the voltage converter may disturb the radio frequency signal if the frequency component in the output voltage of the voltage converter is approximate to the frequency of the radio frequency signal. Therefore, when a load of the voltage converter is an electronic module having a radio frequency circuit, it is desirable to control the signal component (or noise component) capable of disturbing the load in the output voltage of the voltage converter. In other words, the voltage converter is under a low-noise mode through controlling the noise in the output voltage of the voltage converter.
In a voltage converter under low-noise mode, the driving signal of the control element therein can be controlled for reducing the noise component in the output voltage of the voltage converter, so that the control element is relatively smoothly switched from on to off, and/or switched from off to on. For example, the driving signal of the control element can be controlled to improve a cut-off voltage of a triode serving as a control element. When the load (e.g., the resistance of the electronic module) powered by the voltage converter is heavy, the low-noise characteristic of the voltage converter can supply power for the electronic module without disturbing the radio frequency signal of the electronic module as far as possible. However, when the load powered by the voltage converter under the low-noise mode is light and even zero, current flow can still appear on the control element, thereby causing unnecessary power consumption. Therefore, it is desirable to reduce the static current when the voltage converter under the low-noise mode has light load.